Purification method for producing high purity niobium compound and/or tantalum compound

ABSTRACT

An object of the present invention is to provide a method for purifying a highly pure niobium compound and/or tantalum compound, the method enabling the purification of a highly pure niobium compound and tantalum compound in a simplified manner at a low cost. The object is met by providing a method comprising adding an organic solvent to an aqueous solution containing a niobium compound and/or tantalum compound together with impurities, and then performing extraction via the solution. A niobium compound and/or tantalum compound dissolved in a solution is allowed to precipitate, and said aqueous solution is obtained by dissolving the precipitate in water.

TECHNICAL FIELD

The present invention relates to a method for obtaining a highly pureniobium compound and/or tantalum compound.

BACKGROUND ART

Niobium has been used as an additive to steel because niobium iseffective in stabilizing carbon in steel and prevents the progression ofcorrosion among particles. A niobium alloy has been used as a materialof a conductive tube attached to the light emitting portion of ahigh-pressure sodium lamp, a superconductive material and an additive toa super alloy. Recently, demand for niobium oxide is notable becauseniobium has been widely used in electronic and optical fields.Particularly in those fields, highly pure niobium is indispensable.Purification of niobium compounds has been achieved by various methodsdepending on the niobium compounds serving as starting materials. Forexample, the purification of niobium oxide has been achieved bydifferential crystallization, solvent extraction, ion-exchangeresin-based separation, distillation, etc. However, ores used for theextraction of niobium such as columbite or niocalite contain tantalumtogether with niobium. Moreover, since niobium shares many physical andchemical properties with tantalum, it has been extremely difficult toseparate niobium from tantalum. Accordingly, many methods proposedheretofore include extracting metallurgical products containing the twoelements from the ores.

For example, the ore containing both niobium and tantalum is ground to apowder which is treated with an acid mixture comprising hydrofluoricacid and mineral acid, e.g., sulfuric acid, so that niobium and tantalumare dissolved in the acid mixture together with other metal impuritiessuch as iron, manganese, calcium, rare earth elements, etc. The solutionis allowed to contact with an organic solvent such as ketone, ester orether of a lower fatty acid, particularly methyl-isobutyl-ketone, andniobium and tantalum are extracted through the organic phase. Accordingto a second method disclosed in A published Japanese Patent Application,Publication No. S58(1983)-176128, the solution described above isallowed to pass over an F-type anion-exchange resin layer to allowniobium and tantalum to be adsorbed to the resin layer, therebyseparating the two elements from other metal impurities. Then, the twoelements are dissolved in aqueous solutions of hydrofluoric acid andammonium chloride to be recovered later.

Various processes for separating niobium from tantalum have beeninvestigated. One such conventional process consists of separating thetwo elements dissolved in an aqueous solution based on the difference intheir concentrations of salting-out and hydrogen ions. Specifically,when the concentrations of hydrofluoric acid and mineral acidconstituting an acid mixture dissolving niobium and tantalum compoundsare reduced, NbF₇ ²⁻ is converted into NbOF₅ ²⁻ while TaF₇ ²⁻ staysunchanged, and thus it is possible to selectively extract tantalum usingan organic solvent such as methyl-isobutyl-ketone. Alternatively, it isalso possible by a process contrary to the above to extract tantalum andniobium using an organic solvent and then to selectively extract niobiumusing an acidic aqueous solution. However, as discussed in A publishedJapanese Patent Application, Publication No. S62(1987)-158118, theprocess for obtaining a highly pure niobium or tantalum using a ketonesuch as methyl-isobutylketone (MIBK) becomes a publicly known technique.Yet another method is disclosed in A published Japanese PatentApplication Publication No. S64(1989)-31937. The method includes the useof a porous support made of active carbon or polypropylene to which anoxygen-containing organic solvent such as trioctyiphosphine oxide or anorganic solvent chosen from alkylamines in which the alkyl group hasfour or more carbon atoms is attached or linked, and comprises the stepof flowing an acidic aqueous solution containing the two elements overthe support so that only tantalum is adsorbed to the support andseparated from the acidic solution. However, it is difficult to obtain asupport to which an organic solvent is linked. Alternatively, if theorganic solvent is impregnated into a porous support, dissolution of theorganic solvent to the water phase poses a problem. However, the aboveextraction method greatly complicates the process, for example, itrequires multiple extraction steps: the water phase must be washed withan organic solvent in a series of mixing-settling steps.

In view of these problems inherent to the conventional methods, anobject of the present invention is to provide a method for purifying ahighly pure niobium compound and/or tantalum compound in a moresimplified manner at a lower cost than is possible with the conventionalmethods.

DISCLOSURE OF THE INVENTION

The method of the present invention achieves the high purification of aniobium compound dissolved in a solution by employing precipitation andextraction steps.

In the following, a system comprising niobium oxide dissolved in anaqueous solution of hydrofluoric acid is taken as an illustrativeexample. However, the precipitation step is not limited to the systembased on a solution dissolving hydrofluoric acid, but can be appliedsimilarly effectively to other systems based on aqueous solutions ororganic solvents. Needless to say, the system may comprise one chosenfrom various niobates, and it is also possible with this system toobtain a targeted highly pure niobium compound.

Specifically, niobium oxide is dissolved in a hydrofluoric acidsolution, and the solution is cooled to precipitate fluoro niobic acid(H₂NbF₇) or oxyfluoro niobic acid (H₂NbOF₅), thereby achieving theefficientseparation of the niobium compound from metal impurities suchas Fe, Ni, Ti, etc. After this precipitation step, impurities are leftin the solution, and the resulting crystal comprises pure fluoro niobicacid or oxyfluoro niobic acid. Needless to say, in the dissolution andprecipitation steps, a solution system containing hydrofluoric acid towhich sulfuric acid, nitric acid or hydrochloric acid is added may beused. After the dissolution of the acid, the system may be diluted withwater so that the HF concentration can be adjusted as appropriate.Precipitation does not necessarily occur as a result of cooling, but mayoccur as a result of concentration.

The introduction of an extraction step enables the efficient removal oftantalum or a kin element belonging to the same element family thatcontaminates the system. The niobium source is not limited to an aqueoussolution containing a niobium compound but may include a liquid melt ofa crystal of fluoro niobic acid or oxyfluoniobic acid obtained bycrystallization, or crystal of fluotantalic acid. The niobium compoundmay be purified by using, as an extraction solvent, a ketone such asMIBK (methyl-isobutyl-ketone), ester or ether, an organic phosphoruscompound, or an amine. The use of a niobium compound as described abovecan obviate the need for counter extraction, and enables the selectiveremoval of tantalum or a kin element after a single step. Generally,extraction occurs in a liquid-to-liquid extraction system where theweight ratio of the organic phase/water phase is 1 or more. However, itis found that the method of the invention makes it possible to achieveextraction in a liquid-to-liquid system where the weight ratio inquestion is below 1.

The extraction step may occur by using an aqueous solution of a niobiumcompound, or a liquid melt of a crystal of fluoro niobic acid oroxyfluoniobic acid obtained by crystallization, or crystal offluotantalic acid without needing the addition of sulfuric acid for theadjustment of the acidity of the solution.

It is possible to obtain a highly pure niobium compound by treating asappropriate a niobium compound obtained as above as a starting material.For example, a solution containing a niobium compound is subjected,after extraction by precipitation, to alkali treatment to produceniobium hydroxide which is then fired to produce highly pure niobiumoxide. It is also possible to obtain potassium fluoroniobate (K₂NbF₇) byallowing niobium hydroxide to react with potassium fluoride or potassiumcarbonate. It is further possible to obtain highly pure niobium metal bysubjecting the highly pure niobium compound to alkali melting.

It is possible by using such a highly pure niobium compound obtained bythe above method as a starting material to isolate highly pure niobiummetal which can be used as a highly functional material in variousfields including electronics and optics.

BEST MODE FOR CARRYING OUT THE INVENTION

Suitable niobium compounds to be purified by the method of the inventioninclude niobium-containing ores or niobium compounds sold in the marketfor industrial use. A niobium compound containing niobium at about 99%is preferably used as a starting material, because then it is possibleto easily obtain a highly pure niobium compound with a purity of 99.99%(4N) or higher at a sufficiently low cost as to make theindustrialization of this niobium-purification process advantageous.

The present invention provides a method enabling the high purificationof a niobium compound, the method comprising using a solution containinga niobium compound, and applying precipitation and extraction steps tothe solution.

Niobium oxide is used as a starting niobium compound. In one such case,niobium oxide is dissolved in a hydrofluoric acid solution. In thiscase, the resulting aqueous solution of niobium exists as hydrogenfluoroniobate or oxyfluoroniobate. Allowing the niobium compound in thesolution to precipitate makes it possible to separate the niobiumcompound from the solution. The precipitation step enables niobium to becompletely separated from other metal impurities such as Fe, Ni, Ti,etc., as well as tantalum ora kin belonging to the same element family.As a result of the precipitation step, impurities are transferred to theliquid phase while the precipitate is recovered to be purified, whichenables the simplified purification of theniobium compound.

Introduction of an extraction step subsequent to the precipitation stepmakes it possible to obtain a highly pure niobium compound which has apurity of 4N (99.99%) or higher in terms of the weight of niobium oxide.With regard to the extraction step, solvent extraction methods have beenvigorously studied since about 1950, and became publicly known astechniques for producing highly pure niobium and tantalum compounds.Among others, Foos et al. studied solvent-extraction using ten and moreorganic solvents (see Foos, R. A. and Wilhelm, H. A., U.S.A.E.C. ReportISC-694(1954)). They found that ketones exhibit particularlysatisfactory results. The inventive method uses, as well, a ketone as anextraction solvent according to the publicly known technique, but,needless to say, suitable solvents may include esters, ethers, organicphosphorus compounds, amines, etc.

According to the method of the invention, the extraction step occurs ina liquid-to-liquid extraction system where the phase ratio (organicphase/water phase) is below 1, which makes it possible to reduce thenecessary volume of the organic solvent and the cost. Generally, theextraction step requires the addition of sulfuric acid for theadjustment of the pH of the system. However, according to the inventivemethod, the extraction does not need the addition of sulfuric acid, andthus makes it possible to reduce the required amount of alkali which isusually added later to neutralize the system. Specifically, theextraction step allows tantalum to be extracted through the organicphase, and thus a more highly purified niobium compound is leftdissolved in the water phase. Needless to say, it is possible to obtaina highly pure niobium compound by repeating the above-describedextraction step. According to the inventive method, however, it ispossible to reduce the contamination of tantalum or a kin belonging tothe same element family to a level of <0.5 ppm after a single cycle ofthe extraction step.

It is possible to obtain a highly pure niobium oxide by subjecting theaqueous solution of the niobium compound obtained as described above toalkali treatment, thereby producing insoluble niobium hydroxide, and tofire the niobium hydroxide. Needless to say, fluoro niobic acid oroxyfluoro niobic acid, that is, a water-soluble crystal obtained via theprecipitation step may be treated, instead of as a starting material forthe production of highly pure niobium oxide, as an intermediate in theproduction of highly pure niobium compounds such as highly pureniobium-containing salts, e.g., lithium oxyniobate (LiNbO₃), potassiumfluoroniobate (K₂NbF₇), etc., or highly puremetal niobium (Nb), etc.

Needless to say, it is also possible to purify tantalum by the samemethod as the one used for the purification of niobium. For example, itis possible to obtain highly pure tantalum compounds such as tantalumoxide (Ta₂O₅), lithium oxytantalate (LiTaO₃), potassium fluorotantalate(K₂TaF₇), tantalum metal (Ta), or other tantalum-containing salts by thesame method.

EXAMPLES

The present invention will be further illustrated by means of Examplesin which a niobium compound is dissolved in hydrofluoric acid. However,the present invention is not limited to those examples.

Example 1

To a transparent PFA-made 1 L-volume vessel equipped with a stirrer, wastransferred 500 g of 75% HF, to which was added, with stirring for aboutthree hours, 150 g of niobium oxide containing metal impurities. Theresulting mixture was further stirred at room temperature. Then, theinsoluble residue was filtered out, and a solution of niobium inhydrofluoric acid containing niobium at about 300 g/L was obtained.

The solution of niobium in hydrofluoric acid was kept with stirring at−20° C. for two hours, to allow a niobium compound to precipitate. Theprecipitate was recovered by filtration.

All the extractions performed in Example 1 were applied to theprecipitate recovered by filtration. The concentrations of individualimpurities contained in the precipitate were as shown in Table 1.

The extraction consisted of transferring 40 g of the niobium compoundobtained as the precipitate in a 1 L volume reflux extractor and addingthereto 360 g of distilled water or 1N aqueous solution of sulfuricacid.

Next, an organic solvent was added to the system. The organic solventsused included methyl-isobutyl-ketone (MIBK hereinafter) andcyclohexanone. In addition, the added amount of cyclohexanone was variedto alter the weight ratio of the organic phase/water phase (o/w ratiohereinafter). Specifically, the added amount of cyclohexanone was made640 g (o/w ratio=1.6), 160 g (o/w ratio=0.4) or 80 g (o/w ratio=0.2).Then, the system was stirred for 10 minutes and left motionless. Thelower water phase was separated from the upper MIBK or cyclohexanonelayer, and 405 g of the extraction water layer was obtained.

To 40 g of a niobium crystal obtained by the precipitation step wasadded 360 g of 1N aqueous solution of sulfuric acid, which was followedby the addition of 80 g of trioctylphosphine oxide in the form of acrystal. The mixture was heated to 65° C. to dissolve thetrioctylphosphine oxide crystal. The trioctylphosphine oxide layer wasseparated for removal, and the extracting water layer weighing 398 g wasobtained.

An extraction experiment solely dependent on a liquid melt of thecrystal obtained as the precipitate was performed as a comparison to theabove solvent extractions. This extraction was based on the fact thatthe crystal has a melting point of 19° C. First, 40 g of the niobiumcrystal obtained as the precipitate was heated to 30° C. to be melted.

Then, to the melt was added 32 g of cyclohexanone, the mixture wasstirred for 10 minutes, and left motionless. The lower liquid melt layerwas separated from the upper cyclohexanone layer, and 42 g of the Nbextracting liquid melt was obtained.

To the extracting water layer or liquid melt was added 65 g of a 28%aqueous solution of ammonia to allow the niobium compound now turninginto niobium hydroxide to precipitate. The precipitate comprisingniobium hydroxide was recovered by filtration being separated fromammonium fluoride and other impurities.

The niobium hydroxide was fired at 1000° C. in an electric furnace toprovide 13.9 g of niobium oxide. The obtained niobium oxide product wasanalyzed by inductively coupled plasma-atomic emission spectroscopy(ICP-AES), and the impurities it contained were found to be as shown inTable 1.

Table 1 shows that the profile of impurities are the same regardless ofwhether sulfuric acid was added to the system for adjusting the acidityof the system, or distilled water was added instead of sulfuric acid.Generally, extraction is performed with the o/w ratio being set at 1 ormore. However, with the present system, even if the o/w ratio was madebelow 1, the content of impurities could be reduced to the same level asis achieved by the general extraction. Thus, according to the inventivemethod, it is not always necessary to add sulfuric acid to the systemfor the adjustment of its pH. This will help to reduce the amount of analkali which would be required for neutralizing the system after theacidification of the system, and thus to reduce the cost.

Comparative Example 1

As Comparative Example, the extraction step accompanying noprecipitation was performed on an HF solution containing niobium oxide,its impurity removing efficiency was studied, and the result wascompared with Examples in which both the precipitation and extractionsteps were performed. Dependency of the impurity removal on the numberof extraction steps actually performed was also studied.

To a transparent PFA-made 1 L-volume vessel equipped with a stirrer, wastransferred 500 g of 50% HF, to which was added with stirring over aboutthree hours, 150 g of niobium oxide. The resulting mixture was furtherstirred while being heated to 70° C. and circulated. Then, the insolubleresidue which was small in amount was filtered out, and a solution ofniobium in hydrofluoric acid containing niobium at about 300 g/L wasobtained. The concentrations of impurities contained in the solutionused for extraction are shown in Table 2.

To a 1 L volume reflux extractor were transferred 150 g of aniobium-containing hydrofluoric acid solution prepared as above, and a1N aqueous solution of sulfuric acid, to which was added cyclohexane tomake the o/w ratio=0.4 to perform extraction. The system was stirred for10 minutes, and left motionless. The lower water phase was separatedfrom the upper cyclohexanone layer, and an aqueous solution containingNb was obtained as a result of extraction.

In a similar manner, to a 1 L volume reflux extractor was transferred150 g of a hydrofluoric acid solution containing niobium prepared asabove which was followed by the addition of 120 g of a 6N sulfuric acidsolution and then of 550 g of cyclohexanone. This allowed Nb and Ta tobe extracted through the organic phase.

To the organic phase was added a 1N aqueous solution of sulfuric acid ordistilled water to achieve re-extraction at the o/w ratio=0.4 or 1. Thesystem was stirred for 10 minutes and left motionless. The lower waterphase was separated from the upper cyclohexanone layer, and an aqueoussolution containing Nb was obtained a result of extraction.

To the aqueous extraction solution was added a 28% aqueous solution ofammonia, and the niobium compound now turning into niobium hydroxide wasallowed to precipitate. Ammonium fluoride and others were eliminated byfiltration to provide niobium hydroxide.

The niobium hydroxide was fired at 1000° C. in an electric furnace toprovide 25.7 g of niobium oxide. The obtained niobium oxide product wasanalyzed by ICP-AES, and the impurities it contained were found to be asshown in Table 2.

Inspection of the results shown in Table 2 reveals that reduction ofimpurities such as Ti, Fe, etc. was not satisfactory after a singlecycle of extraction, and became notable only after re-extraction.Generally, removal of impurities is improved by repeating the extractionstep, but removal of tantalum or a kin element could not reach a levelTa<0.5 ppm, no matter how many times the extraction step may berepeated.

Example 2

To a transparent PFA-made 1 L-volume vessel equipped with a stirrer, wastransferred 500 g of 50% HF, to which was added 130 g of tantalum oxidecontaining metal impurities with stirring. The resulting mixture wasfurther stirred for about five hours while being heated to 70° C. andcirculated. Then, the insoluble residue was filtered out, and a solutionof tantalum in hydrofluoric acid containing tantalum at about 300 g/Lwas obtained.

The solution of tantalum in hydrofluoric acid prepared as above was keptwith stirring at −20° C. for two hours, to allow a tantalum compound toprecipitate. The precipitate was recovered by filtration.

All the extractions performed in Example 2 were applied to theprecipitate recovered by filtration. The concentrations of individualimpurities contained in the precipitate were as shown in Table 3.

The extraction consisted of transferring 40 g of the tantalum compoundobtained as the precipitate and 360 g of distilled water in a 1 L volumereflux extractor and adding thereto 80 g of cyclohexane (o/w ratio=0.2).Then, the system was stirred for 10 minutes and left motionless. Thelower water phase was separated from the upper cyclohexanone layer, and78 g of the cyclohexane layer was obtained.

To the cyclohexane layer was added 65 g of a 28% aqueous solution ofammonia to thereby allow tantalum to precipitate as tantalum hydroxide.The precipitate was recovered by filtration being separated fromammonium fluoride and other impurities, to provide tantalum hydroxide.

The tantalum hydroxide was fired at 1000° C. in an electric furnace toprovide 12.1 g of tantalum oxide. The obtained tantalum oxide productwas analyzed by ICP-AES, and the impurities it contained were found tobe as shown in Table 3.

Inspection of the results shown in Table 3 indicates that extractioncomprising a single dissolution step and precipitation step readilyenables the production of highly pure tantalum with a purity of 99.99%or more as was observed in the purification of niobium.

As discussed above, the inventive method enables, by includingprecipitation as its first step, the highly efficient removal of allimpurities, and thus can reduce not only load imposed on the extractionsolvent, but also the amount of the solvent required for thepurification process.

TABLE 1 EXAMPLE 1 Crystals obtained via precipitation o-layer Crystalsused Trioctyl- MIBK Cyclohexanone for phosphine extraction oxide w-layer(Crystal 1N H₂SO₄ Distilled Distilled Liquid 1N H₂SO₄ 1N H₂SO₄composition precipitated) extraction water water melt extractionextraction extraction extraction extraction o/w  0.2  0.2  0.2  0.8  0.4 1.6 Number of  1  1  1  1  1  1 extractions Impurity content (ppm) Ta660 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 Ti  5  2  2  2  2  2  2 Fe  10  2  2 2  2  2  2 Ni  2 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 Al  1 <0.5 <0.5 <0.5<0.5 <0.5 <0.5 Sb  <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5The numerals represent weights relative to the weight of Nb₂O₅.

TABLE 2 COMPARATIVE EXAMPLE 1 HF solution (undergoing no precipitation)o-layer Solution Cyclohexanone w-layer composition used for 1N H₂SO₄ 6NH₂SO₄ 6 N H₂SO₄ 6 N H₂SO₄ extraction extraction extraction -> extraction-> extraction -> 1N H₂SO₄ 1N H₂SO₄ distilled extraction extraction waterextraction o/w  0.4 2 -> 0.4 2 -> 1 2 -> 1 Number of extractionsExtraction Extraction -> Extraction -> Extraction -> re-extractionre-extraction re-extraction  1  2  2  2 Impurity content (ppm) Ta 1450140 130 50 180 Ti  820 750  5  5  5 Fe  310 230  5  5  5 Ni  40  35  <1<1  <1 Al  30  15  <1 <1  <1 Sb  15  5  <1 <1  <1The numerals represent weights relative to the weight of Nb₂O₅.

TABLE 3 EXAMPLE 2 Crystals obtained via precipitation o-layer Crystalused for extraction Cyclohexanone w-layer composition (Crystalprecipitated) Water extraction o/w  0.2 Number of extractions  1Impurity content (ppm) Nb 100 <0.5 Ti  5  1 Fe  5  1 Ni  <0.5 <0.5 Al <0.5 <0.5 Sb  <0.5 <0.5The numerals represent weights relative to the weight of Nb₂O₅.

INDUSTRIAL APPLICABILITY

It is possible according to the inventive method to easily purify oresthat contain niobium, or a commercially available niobium compound forindustrial use preferably having a purity of about 99%, therebyproducing a highly pure niobium compound containing niobium at 99.99%(4N) or higher. Thus, the method, even when enlarged to an industrialscale, will ensure the production of highly pure niobium compounds at alow cost.

Needless to say, niobium compound crystals or niobium compounds obtainedafter the crystallization step may be treated as an intermediate in theproduction of other niobium compounds containing niobium at a highpurity. It is possible by using such a highly pure niobium compoundobtained by the above method as a starting material to isolate highlypure niobium metal which can be used as a highly functional material invarious high-quality fields including electronics and optics.

A feature of the inventive method consists in the simplifiedpurification of a niobium compound. The inventive method is roughlybased on three processes, i.e., (1) dissolving process, (2)precipitation process, and (3) extracting process (at least a singlecycle of extraction). The inventive method is particularly characterizedby the precipitation process which serves as a primary treatment forremoving impurities and which will be simply executed even at anindustrial scale. The extracting process of the inventive methodrequires only the small amount of an organic solvent, and may evendispense with the addition of sulfuric acid which would otherwise berequired for the adjustment of the acidity of the system, and thus canreduce the use amount of alkali required for the neutralization of thesystem. Thus, the inventive method can reduce energy consumptionrequired for the solvent-extraction step, but also load imposed onsucceeding steps.

Needless to say, it is also possible to obtain a highly pure tantalumcompound by using the same method as described above.

1. A method for obtaining a highly pure niobium compound, comprising:dissolving a first niobium compound including tantalum as an impurity inan HF solution to form a second solution; precipitating crystalscomprising a second niobium compound from the second solution to formprecipitated crystals and a third solution; separating said precipitatedcrystals from said third solution; dissolving said precipitated crystalsin an aqueous solution or melting said precipitated crystals to obtain amelt; adding an organic solvent to said melt or to said aqueous solutionin which said precipitated crystals are dissolved; conducting extractionwith said organic solvent; and recovering said highly pure niobiumcompound.
 2. A method for obtaining a highly pure tantalum compound,comprising: dissolving a first tantalum compound including niobium as animpurity in an HF solution to form a second solution; precipitatingcrystals comprising a second tantalum compound from said second solutionto form precipitated crystals and a third solution; separating saidprecipitated crystals from said third solution; dissolving saidprecipitated crystals in an aqueous solution or melting saidprecipitated crystals to obtain a melt; adding an organic solvent tosaid melt or to said aqueous solution in which said precipitatedcrystals are dissolved; conducting extraction with said organic solvent;and recovering said highly pure tantalum compound.
 3. The methodaccording to claim 1, wherein said aqueous solution includes an acidicsolution for adjusting the acidity of said aqueous solution.
 4. Themethod according to claim 2, wherein said aqueous solution includes anacidic solution for adjusting the acidity of said aqueous solution. 5.The method according to claim 1, wherein no acidic solution foradjusting acidity is included in said aqueous solution.
 6. The methodaccording to claim 2, wherein no acidic solution for adjusting acidityis included in said aqueous solution.
 7. The method according to claim1, wherein the ratio of the organic phase/water phase during saidextraction is below
 1. 8. The method according to claim 2, wherein theratio of the organic phase/water phase during said extraction isbelow
 1. 9. The method according to claim 1, wherein the first niobiumcompound is niobium oxide.
 10. The method according to claim 2, whereinthe first tantalum compound is tantalum oxide.
 11. The method accordingto claim 1, wherein said organic solvent comprises a ketone, an ester,an ether, an organic phosphor compound or an amine.
 12. The methodaccording to claim 2, wherein said organic solvent comprises a ketone,an ester, an ether, an organic phosphor compound or an amine.
 13. Themethod according to claim 1, wherein said crystals comprising saidsecond niobium compound comprise hydrogen fluoroniobate oroxyfluoroniobate.
 14. The method according to claim 2, wherein saidcrystals comprising said tantalum compound comprise hydrogenfluorotantalate.
 15. The method according to claim 1, wherein saidextraction comprises extracting said tantalum impurity with said organicsolvent from said aqueous solution or said melt.
 16. The methodaccording to claim 2, wherein said extraction comprises extractingtantalum with said organic solvent from said aqueous solution or saidmelt so as to leave said niobium impurity in said aqueous solution orsaid melt.